In measurement or analysis systems, it is often important that the systems be calibrated so that they are externally traceable, preferably to some standard such as those maintained by the National Bureau of Standards. For example, in vibration analysis systems, a multiplicity of channels are utilized to receive data for analysis by the system, and to apply excitation signals to the device under test. For accurate analysis it is important that these channels be calibrated, and preferably in a manner which is traceable to the National Bureau of Standards.
In the past, calibration of such systems has been a laborious, manual, time consuming task because of the multiplicity of measurements at each range setting required to fully calibrate a system. Previously, the expected period over which a calibrated system would maintain its calibrated state, was quite short. This was due in part to the manual nature of the calibration process and the manner and mechanisms employed in calibrating them. For example, previous mechanisms suffered from susceptibility to mechanical vibrations, thermal cycling, oxidation, aging and the like. This resulted in instabilities which shortened the lifetime of the calibrated state.
More specifically, in the past calibration of input measurement channels was typically made by manually adjusting potentiometers, in order to adjust the gain of each channel, and adjusting components within a filter block in order to calibrate the phase shift through each channel. For example, potentiometers at the input to or at the output of a gain block, or the value of feedback resistors within a gain block, were adjusted in order to calibrate the gain of the stage. For phase adjustments, potentiometers or variable capacitors currently are used to adjust the poles and zeros within a high order anti-aliasing filter. Such adjustments cause undesirable changes in amplitude characteristics of the pass band.
Alternatively, rather than calibrate the channel itself, the representation of the signal after passing through the channel, was modified to reflect phase and gain corrections. More specifically, the representation of a signal that is received at the output of a channel is modified in amplitude and phase by correction factors previously determined by characterizing the particular channel for phase and gain deviations from a standard. An example of this approach can be found in U.S Pat. No. 4,162,531 to Rode et al.
As an example, for a particular channel, the gain and phase shift through the channel are measured and retained. If the measured gain differs from the desired gain, a correction factor is determined. This is repeated for the measured phase shift for the channel. When the channel is used to handle an actual signal, the signal is permitted to pass through the channel and, once through the channel, corrected by the phase and gain correction factors. Thus, if the signal at the output of the channel is represented as having an amplitude of X, and a phase shift of .phi., and if the gain correction factor is x and the phase correction factor is .psi., the representation of the signal might be modified to be x * X and .phi.+.psi.. These correction factors must be determined and applied to each range/gain setting of the amplifier.
In the "X2" Series of Structural and Vibration Analysis Systems manufactured by Time Data Systems, Inc., predecessor to a now wholly owned subsidiary of the assignee of the subject application, utilized post sampling adjustments in software to adjust for differences in sampling times between channels. In such a system, samples from different channels were taken by time multiplexing each channel through a single sample and hold and analog to digital converter sampling path. As such, samples from different channels were shifted in time relative to one another by a fraction of the effective sample clock of the sampling path. This relative phase shift was accounted for in the post sampling software processing of the samples by assuming a fixed delay between the samples proportional to 1/f.sub.s, where f.sub.s is the effective sample clock.
In Pease, "DAC lends digital control to phase-shifter", EDN Design Ideas Special Issue, Vol. II, July 21, 1988, pp. 154 and 158, there is discussed digital control of phase in an all-pass phase shifter with unity gain. The circuit disclosed employs a digital to analog converter in a variable resistor mode and three operational amplifiers to provide a variable phase shift without a change in gain.
The following references are directed to gain and phase adjustments signal handling channels: U.S. Pat. Nos. 4,473,797, issued Sept. 25, 1984 to Shiota; Re. 31,750, issued Nov. 27, 1984 to Morrow; 3,654,804, issued Apr. 11, 1972 to Helmuth; and Russian 938053, Bul. 23/23.6.82. The following references are directed to the state of the art in vibration testing systems: U.S. Pat. Nos. 3,659,456, issued May 2, 1972 to Marshall et al.; 3,710,082, issued Jan. 9, 1973 to Sloane et al.; 3,800,588, issued Apr. 2, 1974 to Larson et al.; 3,842,661, issued Oct. 22, 1974 to Marshall et al.; 3,848,115, issued Nov. 12, 1974 to Sloane et al.; 4,297,888, issued Nov. 3, 1981 to Hirai et al.; 4,366,561, issued Dec. 28, 1982 to Klein; 4,493,213, issued Jan. 15, 1985 to Uretsky et al.; 4,513,620, issued Apr. 30, 1985 to Uretsky et al.; 4,513,622, issued Apr. 30, 1985 to Uretsky; and Japanese Patent No. 53-135358 dated Nov. 25, 1978.
It should be apparent that the previous calibration techniques which adjust components in the signal path are highly manual-labor intensive, and do not lend themselves easily to computer control, and that the techniques which modify the representation of the signal once it has passed through the channel can be complex and require that adjustments be made to each measurement taken or signal processed. What is needed is a system which is suitable for implementation under computer control and which is simple, inexpensive, reliable and externally traceable.